Optical maser crystals



April 6, 1965 R. SODEN ETAL OPTICAL MASER CRYSTALS Filed Aug. '7, 1961 FIG.

FIG. 2

CURVE 2 lo-' 0.5 LOGARITHM OF EUROPIUM CONCENTRATION IN VE N TORS RELATIVE EMISSION INTENSITY R. R. SODE/V LG. VAN U/TERT ATTO NE) United States Patent O 3,177,154 ()PTICAL MASER CRYSTALS Ralph R. Soden, Scotch Plains, and Le Grand G. Van

Uitert, Morris Township, Morris County, N..l., assignors to Bell Telephone Laboratories, Incorporated, New

York, N .Y., a corporation of New York Filed Aug. 7, 1961, Ser. No. 129,790 9 Claims. (Cl. 252-3015) This invention relates to single crystal tungstate materials exhibiting fluorescent properties and to devices utilizing such crystals.

Recently, considerable interest has developed in a new class of solid state maser devices in which the stimulated frequency is in the optical or near optical spectrum including the infrared and ultraviolet portions of the electromagnetic spectrum. This spectrum encompasses the wavelength range of from 100 A to 2 A. In principle, these devices are directly analogous to the microwave maser, and the mechanics of their operation are well detailed in the literature, for example as described by A. L. Schawlow and C. H. Townes in U.S. Patent 2,929,922, issued March 22, 1960.

Among the more promising forms of optical masers are those which employ a material whose energy level system is characterized by at least three energy levels, with the separation of these levels falling within the desired operating frequency ranges. During operation, t..ere is established, at least intermittently, a nonequilibrium electron population distribution in a pair of the selected three energy levels. In particular, the population of the higher of the selected pairs of energy levels is increased to the point at which it is greater than that of the lower level. It is customary to refer to a material in such a state of nonequilibrium as exhibiting a negative temperature.

It is characteristic that if there is applied to a material in a negative temperature state a signal of a frequency which satisfies Plancks Law with respect to the two energy levels in nonequilibrium, the applied signal will stimulate the emission of radiation in phase with the signal frequency from the material and the signal will be amplified.

In other words, the active maser material is chosen such that the two energy levels are separated by an energy equal to h where h is Plancks constant and a is equal to the frequency to be amplified. This separation is less than the separation between thetop and bottom levels of the selected three-level energy system.

The negative temperature state is establishedby applying to the material pump energy of a frequency of at least the frequency corresponding tothe separation between the top and bottom levels of the selected three-level energy system. The application of sufiicient pump energy effects electron transitions from the bottom level to the top level and thepopulations of the bottom and top levels are thereby made to approach equality. Under these conditions there will be a negative temperature either between those which comprise a host crystal containing paramagnetic ions from which the stimulated emission occurs.

The host crystal of a material meeting the above-described requirements must be capable of acceptingthe paramagnetic ions in such a way that they are able on excitation to fluoresce with good over-all quantum efiiciency, with as much of the emitted energy as possible concentrated in a single line. To maximum amplificaare useful in optical maser devices.

3,177,154 Fatented Apr. 6, 1965 tion of the signal frequency, the emission line preferably corresponds to a transition to a state other than the ground state such that the single bright emission line is narrow in width.

Since the pump sources typically utilized in optical masers generally exhibit an energy output over a broad frequency spectrum, it is desirable that the paramagnetic ions possess a broad absorption spectrum to facilitate establishment of the negative temperature state. Desirably the paramagnetic ions also exhibit a relaxation time sufficiently long so that the qauntum efficiency for fluorescence is close to unity. Otherwise, the magnitude of the pumping frequency would have to be greatly increased in order to maintain a negative temperature state wherein sufiicient electrons are available in the higher energy level to amplify the input frequency. To ensure a narrow emission line, the energy level widths of the pair of spaced energy levels in the negative temperature state are preferably narrow.

In view of the above-detailed requirements, very few optical maser materials are known to the art. Most published work on optical masers is directed to ruby crystals and calcium fluoride crystals containing small amounts of uranium (III) and Samarium (II). Ruby crystals, however, suffer the disadvantage of requiring high pumping power to establish a negative temperature state. As such, under the usual conditions ruby masers are limited in operation to producing a pulsed beam of coherent light.

As previously discussed, there should be a correspondence between the signal to be amplified and the energy level separation of the masermaterial. Therefore, it is desirable that new maser materials having a range of energy level separations and fulfilling the abovedetailed requirements be developed so that a range .of signal frequencies can be amplified.

In accordance with the present invention it has been discovered that certain lithium tungstate single crystals having a noncubic crystalline lattice of the scheelite structure and containing a trivalent rare-earth ion selected from the group consisting of europium, terbium, and ytterbiurn v More specifically, it has been found that a fluorescent composition of matter having the following empirical formula possesses charac teristics suitable for maser use:

o. o. x X 4 where A is an ion selected from the group consisting of yttrium, lanthanum, and gadolinium, B is a trivalent rare-earth ion selected from the group consisting of europium, terbium, and ytterbium, and x has a value of from 0.07 to 0.5. The subscripts in the above formula signify the relative number of gram atoms of the element indicated which are present and thus are also proportional to the relative number of atoms of each element present in the composition.

The materials of the instant invention emit energy of narrow line width. For example, the line width of Li Y Eu WO associated with an emitting wavelength of approximately 6150 A. is in the order of 7 to 9 cm. at liquid nitrogen temperature. The excited electrons evidence a relaxation time. sufficiently long so that the quantum efiiciency for fluorescence is close to unity.

. Since the ions possess at least three energy levels and electron transitions are to other thanthe ground state,

the establishment of a continuous negative temperature state is feasible and the material is capable of fiuorescing in a continuous beam of coherent light. The invention may be more easily understood b reference to the drawing, in which.

FIG. 1 is a front elevational view of an apparatus utilizing the composition of the invention; and

closed herein.

. by curve 1 of FIG. 2.

FIG. 2,, on coordinates. of relative emission intensity and gram atoms per formula of trivalent europium ion, is a semilog plot showing the dependency of the emission intensityon the concentration of rare earth ion in the material of the invention.

, Referring more particularly to- FIG. 1, there is shown a rod shaped crystal 1 having the composition as dis- Pump'energy is supplied by means of helical lamp 2 encompassing rod 1 and connected to an energy source not shown. Lamp 2 is an ultraviolet lamp having a compact arc of high pressure mercury. Ends 3 and 4 of rod 1 are ground and polished so as to be optically fiat and parallel and are silvered so as to provide reflective layers 5 and 6. As indicated, layer 6 is completely reflecting while layer 5 is only partially reflecting, so permitting the escape of coherent radiation 7 having a Wavelength of approximately 6150 A. for the europium-containing compositions, 10,000 A. for the yt-' terbium-containing compositions and 5450 A. for the terbium-containing compositions of the invention. Rod

1 during operation is preferably maintained in an atmosphere of liquid nitrogen (at a temperature of approximately'79 K.) so as to more readily attain a negativetemperature state.

The spectrum of the pump source including ultraviolet,

light is desirably within the range of 2000 A. to 4200 A.'

Although higher frequencies are suitable, sources of such frequencies are not generally available. It has been found that an ultraviolet light source having a peak of 3660 A. is most advantageous for the present purposes.

Although the expressed range is the range of energy most effective, it is not necessary to use a source having an output restricted to thisrange; For example, a gaseous discharge flash bulb, although emitting white light, nevertheless emits a large amount of energy in the desired spectrum.

Device discussion has been largely in terms of most commonly reported maser design. Although such a disclosed-in the literature and may prove advantageous. All such variations are considered to be Within the scope of the invention.

The efiect of rare-earth ion concentration on the emission intensity of the material of the invention is shown In this figure, the ordinate measures the relative emission intensity of several tungstate compositions containing added europium with the abscissa indicating the logarithm of the europium content of these compositions. eifect of europium inclusions in one compositionof the invention having the empirical formula Although not plotted, measurements indicate that lanthanum and gadolinium give results comparable toyttrium in the lithium-yttrium-europium tungstate crystals. Additional measurements indicate that terbium and ytterbium rare-earth ionsgive results comparable to europium in the above tungstate host crystals.

Basedon FIG. 2, inclusions of 0.07 to 0.5 gram atom per formula of trivalent rare-earth ion in the tungstate host crystal resultin a material exhibiting an enhanced Curve 1 of this figure shows the device is easily fabricated, other configurations have been 7 taining composition isheated in a suitable alkali metal emission intensity. The lower limit of 0.07 gram atom is based onthe necessity of having sufiicient unpaired electrons available in the negative temperature state to As seen from curve adequately amplify the input signal. 1, smallrare-earth ion inclusions result in a sharp decrease in the emission intensity exhibited by the ;materials of the present invention. The upper limit of 0.5 is obtained when all-of the yttrium in the lithium! yttrium-europium 'tungstate crystal has been replaced by europiunr, resulting in a lithium-europium tungstate crystal. As seen from curve -1', increasing-amounts of rare earth ion inclusions above the minimum limit of 0.07 causes the. emission intensity to pass through a maximum tensity.

a tungsten filament lamp, whose output .was assumed to haveablack-body dependence, to give relative values of brightness of the emitting surface in units of power per .unit'wavelength range. Emissionwas. excited by illuminating a sample one inch long-by one-half inch wide by one-quarter inch. deep with a 3660 A. rich H4 spotlamp through a Corning5874 filten. The measurements were made. at room temperatureand the intensities are relative to 100 forthe 6150 A. peak of. a comparable sample of Na Eu WO It has been determined that. replacing I.yttrium, lanthanum and gadolinium with other Group IIIions, for example aluminum, gallium, or scandium, results in a different structure which does, not accommodate appreciable .quantities of the fluorescent rare-earth ion, thereby resulting in a sharp decrease in theemission intensity of the material. It has also been determined that replacing terbium, europium, and ytterbium with other rare-earth ions that are known to fluoresce in other environments, results in a material that-is either nonfiuorescent or only weakly, fluorescent. For example, the substitution of a fraction of a percent of cerium results in a material which is nonfluorescent.

Curve 2 of the figure depicts the-results obtained by replacing lithium in the Li Y EuWO structure with potassium. Such substitution resultsin a material exhibiting the highest emission intensity at potassium-europium tungstate. This compound which has a different ,structure has :an emission intensity only one-fifth as bright as the lowest emission intensity of the material of the invention. In contrast to the mater-ialof the instant invention, yttrium additions to a potassium-europium tungstate structure cause no, enhancement of emission in- Thetungstate crystals oftheinstant invention are adyantageously grown by the ditungstateflux method disclosed inPatent 3,003,112, issued October 3, 1961 to L. G. Van Uitert. Briefly, in :accordance with this method, a mixture of the desired tungstate'and a rare-earth ion-conditungstate flux to a temperature suificient to form a molten solution. The flux, which is a solvent for the tungstate and the rare-earth ion-containing composition, may contain'an excess of tungstic anhydride in a molar amount up to the amount of the tungstate present in the initial mixture to enhance the solubility of these components. "hemolten solution is then slowly cooled until it solidies. containing the desired paramagnetic ion are formed in the flux.

The initial mixture is equivalent to ,10.mol 'parts to mol parts of the tungstateandQOmol parts to 25mol parts of the flux. One advantageof the flux is its solvent power, which permitstemperatures of 900 C. to 1450 C. .to'be used in forming a molten solution of .the mixture. These temperatures avoid reduction of the rare-earth ions to lower undesirable valency states 'which are not suitable for optical maser use. Additionally, loss of tungstic anthe amount of accidentally added rare-earth ion impurity In the. course of cooling, crystals of'the tungstate in order to insure consistent results. For example, the presence of a fraction of a percent of cerium is suflicient to quench the fluorescence of other rare-earth ions in the tungstates. With the exception of cerium, however, accidentally added rare-earth ion impurities are generally tolerated in the composition of the invention in amounts up to 1.0 percent of the principal active rare-earth ion intentionally added. To minimize such contamination, spectroscopically pure rare-earth substances, such as oxides, are typically utilized in the initial mixture. Generally, the nonactive ion impurity limits are not critical and ordinary reagent grade tungstates or substances that react to form the tungstates are utilized.

The atmosphere in which the initial mixture is heated is not critical. However, it is well known to use an oxy- -gencontaining atmosphere such as air, oxygen or oxygen plus an inert gas to prevent an ion in a higher valency state such as europium, which is unstable at elevated temperatures, from being reduced to a lower valency state. Similarly, for convenience, atmospheric pressure is normally used, although pressure is not critical. As is well known, increased pressures in general enhance solubility of the solute, thereby permitting lower temperatures to be used.

After the heating step, the molten solution is cooled at a controlled rate of 0.1 C. per hour to 25 C. per hour in the same atmosphere used in the heating step until it solidifies, forming tungstate crystals having fluorescent rare-earth ions dispersed therein. The solidification point is readily determined visually. For most of the molten solutions, cooling to a temperature of 650 to 850 C. is adequate to cause solidification.

The tungstate crystals in the flux are then furnace cooled or quenched to room temperature. The ditungstate flux is removed from the tungstate crystals by washing the crystals with an alkali such as a solution of sodium hydroxide.

Specific examples of procedures utilized in the preparation of compositions of the invention are given below. In all cases the properties of the resulting compositions were measured as previously described and the measurements plotted in accordance with the description in conjunction with FIG. 2. These examples are to be construed as illustrative only and not as limiting in any way the scope and spirit of the invention.

Example 1 44.5 grams Li CO 276.2 grams W 9.48 grams Y O and 3.7 grams Eu O were dry mixed together. The mixture was then heated in a platinum crucible in air for 16 hours at a temperature of 1120 C. The molten solution formed was then cooled in air at a control-led rate of 2.5 C. per hour to a temperature of 700 C. The resulting solids were then furnace cooled to room temperature and washed with hot sodium hydroxide, leaving lithium-yttrium tungstate crystals doped with trivalent europium. The formed crystals had the following composition:

Example 2 44.5 grams Li CO 276.2 grams W0 5.93 grams Y O and 9.25 grams B11 0 were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-yttrium tungstate crystals doped with trivalent europium. The formed crystals had the following composition:

0.s o.25 o.25 4

Example 3 44.5 grams Li CO 276.2 grams W0 8.55 grams Lil-203 and 9.25 grams Eu O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-lanthanum tungstate crystals doped with trivalent europium. formed crystals had the following composition:-

o.5 o.2s o.25 4

Example 4 44.5 grams Li CO 276.2 grams W0 9.54 grams Gd O and 9.25 grams Eu O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-gadolinium tun-gstate crystals doped with trivalent europium. The formed crystals had the following composition:

Example 5 44.5 grams Li Co 276.2 grams W0 9.48 grams Y O and 4.14 grams Yb O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-ytterbium tungstate crystals doped with trivalent yttrium. The formed crystals had the following composition:

o.5 o.1 o.4 4

Example 6 44.5 grams Li CO 276.2 grams W0 5.93 grams 0 and 10.33 grams Yb O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-ytterbiurn tungstate crystals doped with trivalent yttrium. The formed crystals had the following composition:

o.5 o.25 o.25 4

Example 7 44.5 grams Li CO 276.2 grams W0 8.55 grams La O and 10.33 grams Yb O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-ytterbium tungstate crystals doped with trivalent lanthanum. The formed crystals had the following composition:

Example 8 44.5 grams Li CO 276.2 grams W0 9.54 grams Gd O and 10.33 grams Yb O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-ytterbium tungstate crystals doped with trivalent gadolinium. The formed crystals had the following composition:

Example 10 44.5 grams Li CO 276.2 grams W0 5.93 grams Y O and 9.65 grams Tb O were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of lithium-terbium tungstate crystals doped with trivalent yttrium. The formed crystals had the following composition:

o.5 o.25 o.25 4

Example 11 The 44.5 grams Li CO 276.2 grams W0 8.55 grams La o and 9.65 grams Tb O were dry mixed. The mix ture then underwent the same processing as detailed above, with the resulting formation of lithium-terbium 44.5 grams Li CO 276.2 grams W 9.54 grams Gd O and 9.65 grams Tb O were dry mixed. The

mixture then underwent the same'processing as detailed above, with the resulting formation of lithium-terbium tungstate crystals doped with trivalent gadolinium. The formed crystals had the following composition:

n.5 o.25 o.25 4

Example 1 3 The crystalsformed in accordance with the procedure set forth in Example 1 were melted in a platinum crucible maintained at a temperature of about 1350- C. Another tungstate crystal of the same composition was placed on the end of aplatinum wire and the tip of the crystal was then immersed in the above melt. The crystal was rotated at 60 rpm. and slowly pulled out of themelt at a rate of one inch per hour. At the end of one hour a rodshaped crystal one inch long was formed. The crystal had the same composition as the original seed crystal on which it was, formed.

Example 14 The elongated crystal formed in accordance with the procedure detailed above was partly immersed in a melt having the composition set forth in Example 1. The

crystal was rotated at 60 r.p.m. and slowly pulled out of V the melt maintained at. a temperature of 1000 C. at a rate of one inch per week. At the end of a week, the elongated crystal'had increased in length'by an additional 'inch. The added growth had approximately the same composition as the original crystal on which it was formed.

" What is claimed is:'

1. A compositionof matter-consisting essentially of a single crystal tungstate material having a crystalline lattice of the scheelitestructure, and an'empiricalformula Li A B,;WO ',.where A is an ion selectedfrom the group consisting of" yttrium, lanthanum, and gadolinium, B is a trivalent rare-earth ion selected from the group consisting of europium, terbium, and ytterbium, and x t has a value of from 0.07 to 0.50. I

2. A composition of matter invaccordance with claim 1 wherein x has a' value of from.about 0.l2 to about 0.45. 3. A composition of matter in accordance with; claim 1 wherein x has a value of from about 0.15. to about 0.40. 4.. A' composition ofmatter in' accordance with claim 1 wherein .Ais. yttrium. a

5. A composition of-matter in accordance with claim 4 wherein B is europium.:

6. A composition of'matter wherein A is lanthanum.

7. A composition of matterin accordance with claim 6 wherein B is europium.

8. A composition of matter in'accordance with claim 1 wherein A is gadolinium.

9. A composition .of matter in accordance with claim 8 wherein B. is europiunL. a

in accordance with claim 1 References Cited by the Examiner UNITED STATES PATENTS 1 3/60 Schawlow .88-61 OTHER REFERENCES Kroger: Some Aspects of the Luminescence of Solid,

Elsevier Pub. Co., New York, 1948 '(pp. 109, 286, 290,

291, 293, 295 and 297);

MAURICE'A. 'BRINDISI, Primary Examiner. JOSEPH R. LIBERMAN, Examiner. 

1. A COMPOSITION OF MATTER CONSISTING ESSENTIALLY OF A SINGLE CRYSTAL TUNGSTATE MATERIAL HAVING A CRYSTALLINE LATTICE OF THE SCHEELITE STRUCTURE AND AN EMPIRICAL FORMULA LI0.5A0.5-XBXWO4, WHERE A IS AN ION SELECTED FROM THE GROUP CONSISTIG OF YTTRIUM, LANTHANUM, AND GADOLINIUM, B IS A TRIVALENT RARE-EARTH ION SELECTED FROM THE GROUP CONSISTING OF EUROPIUM, TERBIUM, AND YTTERBIUM, AND X HAS A VALUE OF FROM 0.07 TO 0.05. 